Leanpub makes a separate file for each post it imports. So you can just delete the files you don't want from your Leanpub Dropbox folder, and remove them from the Book.txt file. Or, if your blogging platform supports custom RSS feeds for categories or dates, you can make an RSS feed containing only the posts you want (by category, say) and then import that feed into Leanpub instead of your full feed.